This invention relates to the field of metallurgy and particularly to the field of processing precipitation hardenable aluminum alloys.
A prior art process for producing fine grain (less than about 10 .mu.m) aluminum alloy is described in U.S. Pat. No. 4,092,181. Such process has been used to produce fine grain sheet, bar, forgings, and other forms which are plastically deformed to produce a fine grain throughout their thickness. However, fine grain cannot be readily produced in very thick sections using prior art processes because of the difficulty of providing the necessary deformation. Additionally, most aluminum alloy raw stock is still produced by conventional coarse grain methods.
In some applications it is desirable to have a fine grain at the surface of an aluminum alloy part, but it is not necessary to have a fine grain below the surface. For example in the case of exfoliation corrosion, a fine grain at the surface of a part should provide markedly superior corrosion resistance even through the center of the part is coarse grained. See for example the article "Characterization of Fine-Grained Superplastic Aluminum Alloys" by N. E. Paton, C. H. Hamilton, J. Wart, and M. Mahoney in Journal of Metals pp. 21-27, August 1982.
In some prior art processes the corrosion resistance of aluminum alloy parts is improved by shot peening or otherwise working the surface of the part to achieve a localized compressive stress state. However for high strength precipitation hardenable aluminum alloys, this approach may not be feasible because the protective residual stresses are relieved by the thermal treatments used after shot peening to achieve full strength.
Another prior art technique for improving the corrosion resistance of high strength aluminum alloys is to overage the alloy by heat treatment. Although the overaged material may have improved corrosion resistance, the overaging treatment lowers the strength of the material.
In addition to corrosion resistance, there are other surface-related phenomena which might be affected by grain size at the surface of the material. Such phenomena include surface finish after forming and fatigue crack initiation which originates at the surface. Thus, there is a need for aluminum alloys having a surface with a fine grain structure which is obtainable without a reduction in strength and which is obtainable regardless of the size of the part.